In a transmission network comprising a core network, said core network is usually an optical network due to its superior traffic capacity compared to electrical signaling networks. The optical signals may have to be converted, e.g. to electrical signals, when distributed to Network Terminals (NTs) or local distribution networks in the subscriber neighborhood. In an optical fiber telecommunications network, an Optical Network Unit (ONU) is provided for terminating the optical fiber transmission line and for converting between different transmission technologies, such as Passive Optical Network (PON) and another transmission technology. In other words, the object of the ONU is therefore to adapt the data traffic from one transmission technology to another transmission technology in the transition point between the two technologies.
According to the present standard specification regarding QoS, high priority packets of the incoming data packet traffic are sent before Low priority. Management traffic always has highest priority and Voice over IP traffic has higher priority than normal Internet traffic. Each ONU is therefore provided with a scheduling mechanism to obtain such a differencing function and performance regarding different classed data packets. The scheduling options in the ONU can be more or less advanced. Schemes like Strict Priority, Weighted Round Robin or Deficit Round Robin may be implemented increasing the complexity of the ONU. The ONU's advanced scheduling task is regarded as a necessity by network operators for providing and withhold QoS requirements for the PON network, as the incoming data packets often have to be stored according to QoS priority classification before transmitted in the upstream direction towards the OLT. In the point-to-multipoint Passive Optical Network the upstream bandwidth, i.e. the uplink bandwidth, is shared between multiple Optical Network Units (ONUs). The bandwidth is not necessary shared equally between ONUs connected to an upstream located Optical Link Termination (OLT). Even for a single ONU the bandwidth may vary over time. The PON Uplink interface between ONUs and OLT is supposed to be variable in bandwidth, while the interface between the ONU and NT is fixed at e.g. 1 Gbps. Said PON Uplink is controlled and allocated by the OLT independently of the fixed bandwidth and may therefore be shifted over time to a value below that of the fixed bandwidth. If the uplink bandwidth to the OLT is allocated below 1 Gbps, the ONU must involve means for accommodating received data packets. If the ONU is not provided with storing elements or enough buffering capacity, the ONU will start dropping packets when data packet congestion occurs.